1. Technical Field of the Invention
The present invention relates generally to the manufacturing of small dimension features of objects, such as integrated circuits, using photolithography techniques. More particularly, it relates to a method of printing design features within a photosensitive material using alternating Phase Shift Mask (PSM) technology with a double mask exposure strategy, and to a method of fabricating a mask set comprising a phase shift mask and an associated binary trim mask.
2. Description of Related Art
Photolithography, in the context of manufacturing of semiconductor devices is the process of patterning windows in a layer made of photosensitive material (sometimes referred to as photoresist or resist layer), which layer typically covers a semiconductor substrate. These windows expose small areas in which the semiconductor substrate is modified by a subsequent processing operation. For instance, the manufacturing of CMOS integrated circuits involves several iterations of a process sequence, including the patterning of a respective resist layer, followed by the execution of an etch, implant, deposition, or other similar operation, and then by the removal of the remaining resist to make way for a new resist layer to be applied for another iteration.
Due to the continuous decrease in the size of printed dimensions, tighter design rules are required for laying out small dimensions design features on a semiconductor substrate. More particularly, it is desirable to address the problem of patterning so-called Critical Dimension (CD) design features. The Critical Dimension is the minimum dimension of features allowed by the design rules in a given technology. For example, present 60 nm technology allows achieving lines of 60 nm width as the CD. This CD is usually less than the optical wavelength used to print it inside photoresist.
In this context, various methods for increasing resolution of photolithographic images toward the sub wavelength range have been proposed. One of the techniques known in the current state of the art uses the alternating Phase Shift Mask (Alt PSM, for short) technology. Using the Alt PSM technique allows one to increase the resolution and the contrast of line segments inside the photo resist.
In the context of the Alt PSM technique, a design pattern, e.g., a line segment, is printed by the shift mask when it exhibits a critical dimension (a CD constraint) so that it cannot be oversized and/or when its edges must be located very precisely (a localization constraint). Thanks to this technique, a line segment with as low as a 60 nm width can be defined, and its edges can be located with a spatial precision of little as 3 nm.
A number of Alt PSM techniques already exist, including both a single mask approach as well as double mask approach, either with dual phases or triple phases. The double mask approach, which relies on a double mask exposure strategy, has reached such maturity that commercial software tools are now available in the market place which supports this technique. However, further printing enhancement techniques still need to be developed and implemented to be able, for instance, to print a gate length of around 60 nm after photolithography.
Conventionally, the phase shift mask defines only the central portion width of a line segment. Stated otherwise, it is not suitable for defining the ends of the line segment, which are thus to be defined by a conventional mask, namely the binary trim mask. Besides, the phase shift mask provides a precision of as little as 3 nm on the location of each edges.
For instance, in a number of layout configurations the pattern of the design which is to be printed, e.g., a Poly-Silicon (Poly-Si) line segment, will be defined on one edge by the phase shift mask and on the other edge by the binary trim mask. When, as a result of the density of the design this line segment cannot be upsized, a photolithography process marginality leading a discontinued (interrupted) line can emerge. The reasons for this are the mask-to-mask overlay error, the reticle alignment error and the scanner stage positioning error. The merging of these error contributions can lead to a 15-20 nm fine-width loss. This leads to highly focus sensitive areas of the design to be printed, with the risk of discontinued (interrupted) Poly-Si lines.
There is accordingly a need in the art to further improve the Alt PSM technology, whereby to alleviate the foregoing and other problems.